In typical flue gas desulfurization absorber towers, the tower configuration is columnar thereby resulting in a constant gas flow velocity therethrough. In general, the flue gas velocity only changes at the entrance to and the exit from the tower since such entrances and exits are typically much smaller in cross-section than the cross-section of the tower. Thus, through the reactive areas of the tower, the flue gas velocity remains constant, this velocity value being determined by the cross-section of the tower and the volume of the flue gas flowing therethrough (the design or selection of the cross-section or area of the tower, whether for circular or non-circular geometries being a function of the preferred rate of removal and the desired gas residence time).
However, recent studies have shown that the behavior of the slurry droplets sprayed within an absorber tower will vary depending on the flue gas velocity within the tower. Thus, their behavior will not be the same for all flue gas velocity values. For example, at a droplet mean diameter of from 1000 to 5000 microns and at a flow velocity of about 4-7 feet per second (FPS), the measured spray pressure drop within the tower is less than the pressure drop calculated from the spray nozzle momentum equation. At about 10 FPS, the difference between the measured pressure drop and the calculated pressure drop (this difference being termed "recovery") is virtually zero. Such low levels of recovery permit the droplets to fall under their own weight with the gas droplet contact time being controlled by the relative velocity of the droplet. However, above 10 FPS and dependent on droplet mean size, the measured pressure drop is greater than the calculated pressure drop (a positive recovery value) thereby resulting in the suspension of the droplets in the gas stream as indicated by the greater pressure differential across the spray zone. This suspension increases the contact time between the gas and the droplets.
Furthermore, at this greater flow rate (i.e. about 10 FPS and above), it was also observed that the spray around the spray header/nozzles was thicker or more dense than would normally occur around these headers at lower gas velocities dependent on the droplet mean diameter. In other words, a dense fog of spray droplets became suspended in and about the spray zone.
In addition to the above, data also shows that the removal efficiency of SO.sub.2 decreases progressively as the flue gas velocity within the tower increases. At about 4-10 FPS, the rate of loss in removal efficiency is fairly slow when compared to the rate of loss at velocities greater that about 10 FPS. However, operating at the preferred lower velocities for greater SO.sub.2 removal requires a larger volume absorber tower that will occupy more space at the plant or facility. Thus, operating an absorber tower at about 10 FPS has been deemed to be the most economic velocity from both a capital and operating cost viewpoint and from a SO.sub.2 removal viewpoint.
In absorber towers without a tray or other internal devices therein such as packing or another gas distribution device, any increase in flue gas velocity therethrough so as to accommodate an increase in flue gas volume will naturally result in an increase in system resistance along with a decrease in removal efficiency. In an effort to recoup this lost SO.sub.2 removal efficiency, the operators of the tower will generally either use additives, increase the amount of reagent or otherwise change the chemistry within the tower, or they may increase the liquid/gas ratio within the tower. These steps all result in higher pump power requirements and an increase in the pressure drop across the tower which, in turn, results in an increase in fan power.
In absorber towers having a tray therein, the use of the tray alone is known to have an advantage of from 20 to 35 units with respect to the liquid/gas ratio over spray towers without such a tray. Additionally, a tray tower (or a packed tower) creates a high efficiency removal region in which significant SO.sub.2 removal is achieved. Unfortunately, these tray tower designs incorporate additional system resistance due primarily to the tray itself. Such tray resistance generally comprises two separate elements, the first being the dry tray resistance which is due to the restriction in the flow area. This dry tray resistance does not contribute to the removal efficiency of the pollutants. The second component of tray resistance is the wet tray resistance which is due to the contact between the gas and the liquid layer, including the forth, upon the tray. Such wet tray resistance causes a pressure drop that does contribute to SO.sub.2 removal. Ideally, it would be preferable to increase the wet pressure drop without corresponding and substantial increases in the dry component of the tray resistance.
Consequently, as the flue gas velocity through the tower increases so as to accommodate an increase in flue gas volume, the considerable dry tray pressure drop across the tower also increases without any corresponding increase in SO.sub.2 removal efficiency. This increase in flue gas velocity in the tray tower generally results in higher system resistance that would not normally be found in an open spray tower or one without a tray. In order to regain such lost efficiency, the operators are oftentimes forced to also use additives or increase the liquid/gas ratio thereby also resulting in higher pump power requirements in addition to an increase in fan power needs.
It is thus an object of this invention to utilize such knowledge and configure an absorber tower that more closely conforms to the requirements needed to remove pollutants from a particular flue gas. One object of this invention is to use low gas velocities for the promotion of high removal rates in one region of the tower and to use higher flue gas velocities elsewhere within the tower where such higher velocities can be tolerated. Another object of this invention is to configure an absorber tower having the required amount of residence time at the desired velocity and then to progressively increase this flue gas velocity so as to both enhance SO.sub.2 removal and to boost or increase the passage of the flue gas through the tower thereby making the tower more efficient. Yet another object of this invention is to configure an absorber tower having a first flue gas velocity in the inlet zone and a second flue gas velocity at the spray zone. Still another object of this invention is to configure an absorber tower with yet a third flue gas velocity at the mist eliminators zone and possibly a fourth flue gas velocity at the exit from the tower. These and other objects and advantages of this invention will become obvious upon further investigation.